1. Field of the Invention
This invention relates to a device for controlling the ignition timing for internal combustion engines.
2. Description of the Related Art
There has been widely known a device which detects the knocking of an internal combustion engine and controls the ignition timing of the internal combustion engine relying upon a knocking signal. The knocking is usually detected by using a knocking sensor. If the knocking sensor becomes faulty such as breakage in the line, it becomes impossible for the sensor to detect the knocking. As a result, the ignition timing control of the internal combustion engine no longer works, which sometimes causes the internal combustion engine to be damaged. It is therefore very important to detect the fault in the knocking sensor.
JP-B-6-92748 discloses a device for controlling the ignition timing for internal combustion engines, which is capable of detecting a fault in the knocking sensor. This device is designed to precisely detect the broken line fault in the knocking detector that uses a piezoelectric element. Namely, this device detects the fault by measuring a change in the charging voltage or in the discharging voltage when the voltage is applied to the knocking detector from the ignition timing control device or when the voltage is cut. This is based on a discovery that the knocking detector has a capacity and, hence, there occurs a large difference in the capacity between when the knocking detector is normal and when the line is broken. Namely, when a predetermined voltage is applied to the knocking detector, a value of charging voltage or discharging voltage is measured after the passage of a predetermined period of time, namely the magnitude of the voltage is used for judging the fault of the knocking detector.
In the conventional device for controlling the ignition timing for internal combustion engines as described above, when a predetermined voltage is applied for detecting the fault and the voltage is read after the passage of a predetermined period of time, whether the knocking sensor is normal is judged relying upon the magnitude of the voltage. In the low speed region of the engine, therefore, it is easy to judge whether the knocking detector is normal. In the high speed region, however, it becomes difficult to render the judgement. The reason is that since the mechanical vibration of the engine is increased in a high speed region of the engine, a large distortion in the waveform of the voltage from which the fault is detected occurs, and thereby the comparison concerning their magnitudes between the voltage from which the knocking is to be judged and a reference value becomes difficult for making a distinction between the normal state and the breakage of the line. Therefore, despite the knocking detector is normal, the judgement is often incorrectly rendered to be the breakage of line. Therefore, the level of the knocking signals had to be also used for judging the fault in the high speed region only.